Getting some building work completed - how do I do this?

I need to get some reliable builders to work on my house for a day or two and I have some questions:

  1. I'm a pushover in negotiations - how can I avoid being ripped off?
  2. One job I need doing is outdoors - do I need to be on-hand at all times or can I disappear to work and leave them to it?
  3. How do I even find a reputable builder? I don't want to ask friends for recommendations as it might sour the friendship if anything is not right
  4. Is it worth mentioning to builders that I'm autistic?

Thanks in advance